Dear Sir or Madam,
I'm Li hua who (have)purchased a recording pen from your website days ago . 改为have purchased,使用现在完成时更合理,因为对现在造成了影响.

And now I find some problems about(about-->with) the product .about改成with, with表伴随.
Here are some description about it.（这句话语法没问题,但是不很地道,一般可以省略）
First, the sound quality of the recording pen is not good so that I can't make myself heared clear before it .（so that后面这句我不是很理解你是要表达什么意思,可以向我追问）

Second it is not convenient for me to use even too complex for me .（complex的意思是复杂,用在这里不是很合适,如果要表达使用不方便, it is not convenient for me to use就足够了:)）

Third , there's also some little problem(s) that upset me a lot such as the sudden of powering off, heat from the surface of it and so on .（这句话比较长,不是很熟练的话建议拆成短句.such as后可改为：for example, it sometimes powers off suddenly, and the surface of it is often very hot.）
I hope we can check with these for a good solution . Could I (send it )back it to your (your-->you)or could you repair it for free ? I believe your attitude can satisfy us consumer .（back是介词,不能直接用哒~）
Best wishes